Hi, Mike. As another American, I'm not 100% sure but I think I've usually heard it used (in countries that actually speak "English") as a rotating or sliding pin as in a pin or cylinder with a hole for something like a throttle cable in a fork or similar actuating arm. When the cable is pulled (or pushed) in a straight line, the fork or similar arm attachment is pulled in a tangential direction. The trunnion pin rotates to keep the cable pull direct. As a more concrete example, I believe that the pivot on a drum rear brake (the drilled cylinder fitting that the adjusting threaded rod passes through) is sometimes called a "trunnion".